Under-oxidized burners are used for H.sub.2 and/or H.sub.2 +CO mixed gas stream generators, which can in turn be used in a variety of applications including, but not limited to, on-site H.sub.2 equipment, fuel cell power systems, and low emissions combustion equipment. These generators have been described in the patent literature and are typically fed any hydrocarbon fuels.
The present applicant has described in a number of patents various technologies for under-oxidized burners (UOB.TM.) where fuel and air are introduced to and mixed thoroughly in an internal combustion chamber. By providing a mixing device including baffle and wall arrangements against which the fuel and air are caused to impinge, substantially complete air and fuel mixing may be facilitated prior to and during the combustion. In combination with these mixing innovations, preheating of the incoming feed gases is taught to take advantage of the energy in the high temperature exhaust product gases, and thereby raise the non-catalyzed reaction temperature sufficiently that equilibration is approached without carbon as an undesirable side product.
The applicant has, in prior patents obtained, described a number of embodiments of both burner chambers, injector arrangements, and preheating configurations that achieve thorough mixing, igniting, burning, and exhausting of the fuel and/or fuel/air mixtures. These embodiments are prerequisites to the successful production of maximum hydrogen from the fuels. Applicant's U.S. Pat. Nos. 5,207,185, 5,299,536, 5,441,546, 5,437,123, 5,529,484, and 5,546,701 all describe different systems for under-oxidized burners, injectors, preheat heat-exchanger configurations, and other advances in the technology. All of the aforementioned patents are hereby incorporated into this application by reference.
A typical UOB.TM. apparatus disclosed in the prior art has a hydrogen generator including a housing and combustion chamber. Lines for introduction of fuel and air respectively are provided to an injector, which introduces the air and fuel into the chamber. The air and fuel is mixed in a tube or within the chamber and is directed to a baffle against which the fuel and air impinge, reversing the flow of the air/fuel mixture to further enhance mixing. In addition, this prior art also teaches the use of the reaction chamber walls for reversing the flow of the air/fuel mixtures and changing the flow paths to enhance mixing both prior to and during the combustion process. It also teaches means to achieve mixing by aerodynamic effects between adjacent streams of fuel and air. The mixture is then ignited, often initially by a spark plug.
In variations, fuel and air may be introduced through combined or separate lines and passed through a tube or coil or shell-in-shell heat exchanger configuration within the chamber and/or the primary chamber's exhaust in order to preheat the incoming feed gases. This is done before it reaches the injector and enters the primary combustion chamber, and provides recuperative heat exchange needed for achieving high combustion temperatures for effective combustion without a catalyst. If liquid fuels are used, the fuel can be vaporized outside the preheat zone or within the preheat zone.
Carbon will form in the combustion chamber if the feed proportions are such that insufficient oxygen is provided to react with all the carbon in the fuel molecule, such as a stoichiometric ratio with air of less than 0.25 with methane (CH.sub.4). This may be eliminated or substantially reduced by proper formulation and control of the fuel and air feed rates within the under-oxidized burner. Carbon may also form as an undesirable byproduct if the combustion is inefficient or incomplete. This may essentially be eliminated by the feed mixing and preheating methods discussed above and covered in the prior art. Carbon may also form by the disproportionation of CO into CO.sub.2 and carbon, EQU 2CO (gas)=C (solid)+CO.sub.2 (gas)+heat. (1)
Since CO is one of the major products in the UOB.TM., careful design of the burner is needed to avoid this unfavorable side reaction. The relative partial pressure of CO.sub.2 and CO and the temperature of the gases or surfaces in contact with gases control this gas phase reaction. The H.sub.2 O partial pressure is also involved, due to the shift reaction, EQU CO (gas)+H.sub.2 O (gas)=CO.sub.2 (gas)+H.sub.2 +heat, (2)
which effects the partial pressures of CO.sub.2 and CO. Thus, a concise prediction of the formation of C downstream of the main combustor requires knowledge of all three gases. Nevertheless, CO will not disproportionate if the temperatures are high or if sufficient CO.sub.2 is present.
Carbon can also form in processes downstream of the burner system, where the product gases are cooled for various purposes. One of these is in producing further H.sub.2 through the shift reaction, in which typically the UOB.TM. product gases are pre-cooled and humidified. Here, at intermediary temperatures, considerations of the types already discussed show that CO disproportionation, to produce carbon, may occur.